3 Vertebrates-High

Vertebrates Written by Kira Freed

Introduction Touch your elbow. Feel along the middle of your back. The hard objects you feel inside your body are bones. Bones hold up your body and protect organs such as your brain and heart. Scientists classify animals into two groups. Vertebrates are animals with backbones. Invertebrates are animals without backbones. As you read this book, you will find out what the five groups of vertebrates have in common, as well as what makes each group different.

A Closer Look at Bones All bones are made of a hard type of body tissue. (Skin and muscles are other types of tissue.) Without bone tissue, your body would just be a big, soft blob. If you've ever seen a bone, you may think it looks dry and lifeless. But the bones inside your body are alive! Some bone cells help to repair broken bones. Others make blood cells. These bone cells are located in bone marrow, a special tissue in the center of some larger bones. Blood travels in and out of living bone tissue, bringing oxygen and other nutrients and taking away waste products. Bones also have nerve tissue, especially in the outer layer, which is why it hurts so much if you break a bone.

The skeleton is the strong framework that supports and protects a vertebrate's body. The spine, or backbone, is the special feature of vertebrates. The spine is made up of a series of bones, called vertebrae (VER-teh- bray), which are attached to one another in a way that allows the backbone to bend. At the top of the spine is a hard bony case called the skull. It holds and protects the body's control center— the brain. The human skeleton has 206 bones. Other vertebrates have different numbers of bones. Snakes are vertebrates without arms or legs. Their skeletons are made up of a skull and a very long backbone with ribs attached to it.

The ends of many bones are covered with softer tissue called cartilage. Cartilage is rubbery tissue that keeps the hard surfaces of bones from grinding against each other. Other parts of your body have cartilage, too. Feel your nose and ears, which are made of cartilage. Joints are the places where bones meet. Joints allow vertebrates to move in different ways. Some joints, such as your knees, only bend in one direction. Other joints, such as your shoulders, rotate. Still others twist and turn. Cartilage helps all these joints to move smoothly.

Different bones of vertebrates serve different purposes. Leg bones are designed to support the weight of an animal while it walks or runs. Most flying vertebrates have hollow, lightweight bones for greater ease of flight. Some smaller bones allow animals to move their feet, hands, flippers, or wings in precise ways to steer their bodies or handle objects such as food.